Cardiovascular diseases are the leading cause of death in the U.S., accounting annually for more than one million deaths. Atherosclerosis is the major contributor to coronary heart disease and a primary cause of non-accidental death in Western countries (Coopers, E. S. Circulation 1993, 24, 629-632; WHO-MONICA Project. Circulation 1994, 90, 583-612). Considerable effort has been made in defining the etiology and potential treatment of atherosclerosis and its consequences, including myocardial infarction, angina, organ failure and stroke. Despite this effort, there are many unanswered questions including how and when atherosclerotic lesions become vulnerable and life-threatening, the best point of intervention, and how to detect and monitor the progression of lesions.
It is well-documented that multiple risk factors contribute to atherosclerosis. Such risk factors include, e.g., hypertension, elevated total serum cholesterol, high levels of low density lipoprotein (LDL) cholesterol, low levels of high density lipoprotein (HDL) cholesterol, diabetes mellitus, severe obesity, and cigarette smoking (Orford et al. Am. J. Cardiol. 2000, 86 (suppl.) 6H-11H). To date, treatment of atherosclerosis has been focussed on lowering cholesterol levels and modifying lipids. However, recent studies have indicated that 40% of deaths due to coronary disease occurred in men with total cholesterol levels of below 220 mg/dl. (Orford et al. Am. J. Cardiol. 2000, 86 (suppl.) 6H-11H).
In atherogenesis, elevated plasma levels of LDL lead to the chronic presence of LDL in the arterial wall. The modified LDL activates endothelial cells, which attract circulating monocytes (Orford et al. Am. J. Cardiol. 2000, 86 (suppl.) 6H-11H). These monocytes enter the vessel wall, differentiate into macrophages, and subject the modified lipoproteins to endocytosis through scavenger receptor pathways. This unrestricted uptake eventually leads to the formation of lipid-filled foam cells, the initial step in atherosclerosis. If the macrophage is present in an environment that is continually generating modified LDL, it will accumulate lipid droplets of cholesteryl esters, continuing until the macrophage dies from its toxic lipid burden. The released lipid then forms the acellular necrotic core of the atherosclerotic lesion. Subsequent recruitment of fibroblasts, vascular smooth muscle cells, circulating monocytes, and T-lymphocytes complete the inflammatory response and the formation of the mature atherosclerotic plaque. Macrophage-derived foam cells are concentrated in the shoulders of plaques, where their secreted proteinases and collagenases may contribute to plaque rapture, which may lead to a fatal thrombotic event.
The progression of coronary atherosclerotic disease can be divided into five phases (Fuster et al. N. Engl. J. Med. 1992, 326, 242-250). Phase 1 is represented by a small plaque that is present in most people under the age of 30 years regardless of their country of origin. Phase 1 usually progresses slowly (types I to III lesions). Phase 2 is represented by a plaque, not necessarily very stenotic, with a high lipid content that is prone to rupture (types IV and Va lesions). The plaque of phase 2 may rapture with a predisposition to change its geometry and to the formation of mural thrombus. These processes, by definition, represent phase 3 (type I lesion), with a subsequent increase in stenosis, possibly resulting in angina or ischemic sudden death. The mural and occlusive thrombi from plaques of phases 3 and 4, by being organized by connective tissue, may contribute to the progression of the atherosclerotic process represented by severely stenotic or occlusive plaques of phase 5 (type Vb and Vc lesions). The severely stenotic plaques of phase 5, by a phenomenon of stasis and/or deendothelialization, can become complicated by a thrombus and/or myopreliferative response, also leading to an occlusive plaque of phase 5. Two-thirds of coronary occlusions are the result of this late stenotic type of plaques and are unrelated to plaque rupture. Unlike the rupture of less-stenotic lipid-rich plaques which leading to occlusion and subsequent infarction or other acute coronary syndromes, this process of occlusion from late stenotic plaques tends to be silent because the proceeding severe stenosis and ischemia enhance protective collateral circulation (Fuster et al, N. Engl. J. Med., 1992, 326, 242-250).
The ability to detect, quantitate, and monitor atherosclerotic plaque formation is of major clinical importance owing to the progression of these plaques to stable coronary artery disease or to the occurrence of acute ischemic syndromes caused by the rupture of vulnerable plaque. Various imaging modalities for the detection of atherosclerotic lesion and thrombosis associated with plaque rupture have been reviewed (Vallabhajosula, S. and Fuster, V. J. Nucl. Med. 1997, 38, 1788-1796; Marmion, M. and Deutsch, E. J. Nucl. Biol. Med. 1996, 40, 121-131; Cerqueira, M. D. Seminars Nucl. Med. 1999, 29, 339-351; Narula, J. J. Nucl. Cardiol. 1999, 6, 81-90; Narula, J. Nucl. Med. Commun. 2000, 21, 601-608; Meaney et al. J. Magn. Reson. Imaging 1999, 10, 326-338; Knopp et al. J. Magn. Reson. Imaging 1999, 10, 314-316; Goyen et al. Eur. J. Radiol. 2000, 34, 247-256; Becker et al. Eur. Radiol. 2000, 10, 629-635).
Several invasive and noninvasive techniques are routinely used to image atherosclerosis and to assess the progression and stabilization of the disease. These include coronary angiography, intravascular ultrasound angioscopy, intravascular magnetic resonance imaging, and thermal imaging of plaque using infrared catheters. These techniques have been successfully used to identify vulnerable plaques. However, these techniques are generally invasive.
Soluble markers, such as P-selectin, von Willebrand factor, Angiotensin-converting enzyme (C146), C-reactive protein, D-dimer (Ikeda et al. Am. J. Cardiol., 1990, 65, 1693-1696.), and activated circulating inflammatory cells are found in patients with unstable angina pectoris, but it is not yet known whether these substances predict infarction or death (Mazzone et al. Circulation, 1993, 88, 358-363.). It is known, however, that the presence of these substances cannot be used to locate the involved lesion.
Temperature sensing elements contained in catheters have been used for localizing plaque on the theory that inflammatory processes and cell proliferation are exothermic processes. For example, U.S. Pat. No. 4,986,671 discloses a fiber optical probe with a single sensor formed by an elastic lens coated with light reflective and temperature dependent material over which is coated a layer of material that is absorptive of infrared radiation. Such devices are used to determine characteristics of heat or heat transfer within a blood vessel. The devices measure such parameters as the pressure, flow and temperature of the blood in a blood vessel. U.S. Pat. No. 4,752,141 discloses a fiberoptic device for sensing temperature of the arterial wall upon contact. However, discrimination of temperature by contact requires knowing where the catheter is to be placed. These techniques using catheters or devices are invasive, and sometimes may result in or trigger plaque formation or rupture.
An angiogram simply reflects luminal diameter and provides a measure of stenosis with excellent resolution. An angiogram, however, does not image the vessel wall or the various histopathological components. Nevertheless, this technique has become the mainstay of the diagnosis of coronary, carotid, and peripheral artery lesions (Galis et al, Proc. Acad. Sci. USA, 1995, 92, 402-406; Ambrose, J. A. In: Fuster, V. (Ed.). Syndromes of Atherosclerosis: correlations of clinical imaging and pathology. Armonk, N.Y.: Futura Publishing Company, Inc., 1996, 105-122; Kohler, T. R. In: Fuster, V. (Ed.). Syndromes of Atherosclerosis: correlations of clinical imaging and pathology. Armonk, N.Y.: Futura Publishing Company, Inc., 1996, 205-223; Dinsmore, R. E. and Rivitz, S. M. In: Fuster, V. (Ed.). Syndromes of Atherosclerosis: correlations of clinical imaging and pathology. Armonk, N.Y.: Futura Publishing Company, Inc., 1996, 277-289), and is the “gold standard” for anatomic diagnosis despite limited specificity and sensitivity.
An angiogram may be useful for predicting a vulnerable plaque, since low-shear regions opposite flow dividers are more likely to develop atherosclerosis (Ku et al. Atherosclerosis 1985, 5, 292-302). However, most patients who develop acute myocardial infarction or sudden death have not had prior symptoms, much less an angiography (Farb et al. Circulation 1995, 93, 1701-1709). Certain angiographic data have revealed that a regular plaque profile is a fairly specific, though insensitive, indicator of thrombosis (Kaski et al. Circulation 1995, 92, 2058-2065). Such plaques are likely to progress to complete occlusion, while others are equally likely to progress, but less often reach the point of complete occlusion (Aldeman et al. J. Am. Coll. Cardiol. 1993, 22, 1141-1154). Those that do abruptly progress to occlusion actually account for most myocardial infarctions (Ambrose et al. J. Am. Coll. Cardiol. 1988, 12, 56-62; Little et al. Circulation 1988, 78, 1157-1166). One of the major limitations of angiography is that diffuse atherosclerotic disease may narrow the entire lumen of the artery, and as a result, angiography underestimates the degree of stenosis.
The size of the plaque occlusion is not necessarily determinative. Studies show that most occlusive thrombi are found over a ruptured or ulcerated plaque that is estimated to have produced a stenosis of less than 50% of the vessel diameter. Such stenoses are not likely to cause angina or result in a positive treadmill test. In fact, most patients who die of myocardial infarction do not have three-vessel disease or severe left ventricular dysfunction (Farb et al. Circulation 1995, 93, 1701-1709).
Angioscopy is another technique for the visualization of artery walls, rather than the lumen, and for the characterization of atherosclerotic disease. The angioscopy technique reveals the plaque and surface features not seen by angiography. In addition, it allows the observation of the color (red, white or yellow) of the material in the artery, and is therefore highly sensitive for the detection of thrombus. However, it views only the lesion surface and is not representative of the internal heterogeneity of the plaque. As a routine clinical tool, it may not be practical due to the thickness of the catheter and the invasiveness of this technique. U.S. Pat. No. 5,217,456 and U.S. Pat. No. 5,275,594 disclose the use of light that induces fluorescence in tissues, and of laser energy that stimulates fluorescence in non-calcified tissues. These types of devices differentiate healthy tissue from atherosclerotic plaque, but are not clinically useful for differentiating vulnerable plaque from less dangerous, stable plaque.
High-resolution, real-time B-mode ultrasonography with Doppler flow imaging (Duplex scanning) has merged as one of the best modalities for visualization of carotid arteries (Patel et al. Stroke 1995, 26, 1753-1758). Measurements of wall thickness and quantitative analysis of plaque mass and area can be determined. The echogenicity of the plaque reflects plaque characteristics; echoluscent heterogeneous plaque is associated with both intraplaque hemorrhage and lipids, whereas echodense homogeneous plaque is mostly a fibrous plaque. In addition, the configuration of the plaque (mural versus nodular) can identify active (mural) lesions that are more prone to proliferation and thromboembolism (Weinberger et al. J. Am. Med. Assoc. 1995, 12, 1515-1521). Because the technique is not invasive, it can be used to evaluate the efficacy of drug treatment and to study the natural history of atheroscolerosis (longitudinal studies) by follow-up of individuals at increased risk of atherosclerosis. In coronary and peripheral arteries of low extremities, however, Duplex scanning is clinically not as useful as the traditional angiography.
Atherosclerotic calcification is an organized and regulated process and is found more frequently in advanced lesions, although it may occur in small amount in early lesions (Erbel et al. Eur. Heart J. 2000, 21, 720-732; Wexler et al. Circulation 1996, 94, 1175-1192). There is a strong association between coronary calcium and obstructive coronary artery disease, and is clearly shown that the amount of coronary calcium was a useful predictor of the extent of coronary artery disease (Agatson et al. J. Am. Coll. Cardiol. 1990, 15, 827-832; Schmermund et al. Am. J. Cardiol. 2000, 86, 127-132; Budoff et al. Am. J. Cardiol. 2000, 86, 8-11). MRI, fluoroscopy, electron beam CT (EBCT), and helical CT can identify calcific deposits in blood vessels. However, only EBCT can quantitate the amount or volume of calcium (Wexler et al. Circulation 1996, 94, 1175-1192). In addition, the EBCT images of the myocardium can be obtained in 0.1 sec. Because of the rapid image acquisition time, motion artifacts are eliminated (Brundage et al. In: Fuster, V. (Ed.). Syndromes of Atherosclerosis: correlations of clinical imaging and pathology. Armonk, N.Y.: Futura Publishing Company, Inc., 1996, 417-427). It has been well-documented that the presence of coronary artery calcium, detected by EBCT, may be a sensitive early marker for the presence and progression of atheroclerotic lesion before the development of complicated lesions (Janowitz et al. Am. J. Cardiol. 1993, 72, 247-254).
A major limitation using EBCT for the characterization of calcium in the plaque is reproducibility (Becker et al. Eur. Radiol. 2000, 10, 629-635). In particular the reproducibility of small and very small calcium scores (<100) is lower than that for higher score values. In addition, coronary calcium screening can not reveal atherosclerotic plaque that has little or no calcification-and such soft, lipid-rich plaques are perhaps the most dangerous of all, vulnerable to rupture as a result of hemodynamic stress or inflammation (Carrington, C. Diagnostic Imaging, 2000, (April), 48-53; Doherty et al. Am. Heart J. 1999, 137, 806-814).
As red blood cells and platelets gather at the site of the rupture, a blood clot forms and blocks the artery, causing a heart attack. Biologically, calcium may not be the ideal marker because a calcified lesion is presumably a stable lesion, less prone to rupture. More recent data show that coronary calcium scores do not seem to predict myocardial perfusion deficits, plaque burden, or cardiovascular events (Rumberger, J. A. Circulation 1998, 97, 2095-2097; Polak, J. F. Radiology 2000, 216, 323-324).
Magnetic resonance techniques using gradient echo methods to generate images of flowing blood as positive contrast within the lumen of vessels are similar to conventional angiography techniques (Doyle, M. and Pohost, G. In: Fuster, V. (Ed.). Syndromes of Atherosclerosis: correlations of clinical imaging and pathology. Armonk, N.Y.: Futura Publishing Company, Inc., 1996, 313-332; Grist, T. and Turski, P. A. In: Fuster, V. (Ed.). Syndromes of Atherosclerosis: correlations of clinical imaging and pathology. Armonk, N.Y.: Futura Publishing Company, Inc., 1996, 333-362). Magnetic resonance angiography (MRA) of coronary arteries is currently under development, and the resolution is within the range of 1 mm3. MRA techniques provide images of the vessel lumen, whereas MRI studies are often performed to evaluate the effects of the disease on the tissue supplied by the vessel. Recent developments in high-resolution (0.4 mm), fast spin-echo imaging and computer processing techniques visualize in vivo, atherosclerotic plaque activity and intimal thickening (Yuan et al. J. Magn. Reson. Imaging 1994, 4, 43-49).
In a recent clinical study in patients with carotid atherosclerosis, MRI was the first non-invasive imaging modality to allow the discrimination of lipid cores, fibrous caps, calcification, normal media, adventia, intraplaque hemorrhage, and acute thrombosis (Toussaint et al. Atheroscler. Thromb. 1995, 15, 1533-1542; Toussaint et al. Circulation 1996, 94, 932-938). The key advantage of contrast-enhanced rapid imaging techniques is the ability to provide detailed “functional information” with high accuracy (McVein, E. R. Magn. Reson. Imaging 1996, 14, 137-150; Glover, G. D. and Herfkins, R. J. Radiology 1998, 207, 289-235).
In the last two decades, many radiotracers have been developed based on several molecules and cell types involved in atherosclerosis. The potential utility of these radiotracers for imaging atherosclerotic lesions has been studied in animal models, and has been recently reviewed (Vallabhajosula, S. and Fuster, V. J. Nucl. Med. 1997, 38, 1788-1796; Cerqueira, M. D. Seminars Nucl. Med. 1999, 29, 339-351; Narula, J. J. Nucl. Cardiol. 1999, 6, 81-90; Narula, J. Nucl. Med. Commun. 2000, 21, 601-608). In general, radiolabeled proteins and platelets have shown some clinical potential as imaging agents of atherosclerosis, but due to poor target/background and target/blood ratios, these agents are not ideal for imaging coronary or even carotid lesions. Radiolabeled peptides, antibody fragments and metabolic tracers like FDG appear to offer new opportunities for nuclear scintigraphic techniques in the noninvasive imaging of atherothrombosis. However, noninvasive imaging of atherosclerosis remains a challenge for nuclear techniques mainly due to their intrinsic shortcomings, such as low resolution, compared to MRI and CT.
Most of these techniques identify some of the morphological and functional parameters of atherosclerosis and provide qualitative or semiquantitative assessment of the relative risk associated with the disease. Knowledge of the composition of an atherosclerotic plaque may provide a window on the progression of the lesion, which may result in the development of specific therapeutic strategies for intervention. However, these diagnostic procedures are either invasive or yield little information on the underlying pathophysiology such as cellular composition of the plaque, and biological characteristics of each component in the plaque at the molecular level.
As such, a non-invasive method to diagnose and monitor various cardiovascular diseases (e.g., atherosclerosis, vulnerable plaque, coronary artery disease, renal disease, thrombosis, transient ischemia due to clotting, stroke, myocardial infarction, organ transplant, organ failure and hypercholesterolemia) are needed. The non-invasive method should yield information regarding the underlying pathophysiology of the plaque, such as the cellular composition of the plaque and biological characteristics of each component in the plaque at the molecular level.